The reason for the age-related decrease in bifidobacteria numbers is still not well understood. Adhesion to the intestinal mucosa is regarded as a prerequisite for colonization by microbes and induction of the healthy promotion by them. It has therefore been proposed as one of the selection criteria for probiotic strains (42-45).
Ouwehand and coworkers (46) tested four Bifidobacterium strains for adhesion to mucus isolated from subjects of different age groups including healthy newborns, 2- and 6-month-old infants, adults (25-52 years) and elderly (74-93 years). The tested bifidobacteria adhered less to the mucus isolated from the elderly subjects compared to those from healthy infants and adults. The results suggest that the physiological condition of the mucus could be altered by aging, which can reduce the affinity spectrum of the mucus to bifidobacteria from various origins. This may be a factor involved in the decreased colonization of the elderly subjects by bifidobacteria and fewer species of Bifidobacterium present.
Twenty-four Bifidobacterium strains were examined for their ability to bind to immobilized human and bovine intestinal mucus glycoproteins (47). Each of the tested bacteria exhibited its characteristic adhesion to human and bovine fecal mucus. No significant differences were found among the taxonomic species. Among the tested bacteria, B. adolescentis, B. angulatum, B. bifidum, B. breve, B. catenulatum, B. infantis, B. longum and B. pseudocatenulatum adhered to human fecal mucus better than bovine fecal mucus, while the binding of B. animalis and B. lactis was not preferential. These results suggest that the mucosal adhesive properties of bifidobacteria may be a strain dependent feature, and the mucosal binding of the human bifidobacteria may be more host specific.
Fifty-one Bifidobacterium strains were isolated from the feces of healthy adults (30-40 years old) and seniors (older than 70 years of age) and were tested for their ability to adhere to the mucus isolated from the healthy adults (30-40 years of age) (38). The strains isolated from healthy adults, and especially B. adolescentis, bound better to intestinal mucus than those isolated from seniors. These results indicate that the bifidobacteria isolated from the healthy elderly may pose a reduced affinity to the intestinal mucus from healthy adults. These results suggest that the poor colonization of bifidobacteria in the intestinal microbiota of the elderly may also be related to the development of a less adherent Bifidobacterium population as well as the reduced ability of mucus from this age group to facilitate Bifidobacterium adhesion.
Laine and coworkers (48) investigated 30 Bifidobacterium strains isolated from the feces of the healthy elderly (>80 years of age) Japanese and Finnish subjects. These strains were tested for their ability to adhere to the mucus only isolated from their own feces. The better mucus adhesion was observed in the combination of Bifidobacterium from the elderly and their fecal mucus rather than that of probiotic bifidobacteria from adults and the mucus from the elderly.
The enhanced adhesion of B. adolecentis from the elderly to their mucus may, at the least, partly explain that B. adolescentis is a predominant species in the fecal Bifidobacterium microbiota. Therefore, there may be an advanced symbiotic relationship between B. adolescentis and the elderly. The replacement of the predominant species of bifidobacteria upon aging of the host may be one of the important events by which the intestinal microbes affect the homeostasis of physiological functions on the basis of the important contribution of bifidobacteria to human health and well being.
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